Men who have sex with men (MSM) are at high risk for HIV transmission and acquisition. MSM have a high prevalence of an asymptomatic genital tract inflammatory syndrome, called leukocytospermia (LCS), characterized by elevated concentrations of polymorphonuclear leukocytes (?106/ml), proinflammatory cytokines and HIV host cells in semen. We and others have shown that LCS is highly associated with seminal HIV shedding. Although it is widely suspected that LCS has an infectious origin, etiologic agents have not been conclusively identified. We propose to use archived semen samples from a recent study on HIV in semen of sexually active MSM on HAART to identify specific microbiota that are associated with LCS, proinflammatory cytokines, and seminal HIV shedding. We hypothesize that the high prevalence of LCS and HIV in semen of MSM is attributable to colonization of the male genital tract with proinflammatory anaerobic bacteria of rectal origin. We will also compare the microbiome and inflammation markers in prospectively collected semen and other genital samples from sexually active HIV-uninfected MSM and heterosexual men to further establish a relationship between MSM sexual behavior, microflora and inflammation, and to address the possibility that inflammatory microflora increase the risk of HIV acquisition in uninfected MSM. The goal of this project is to provide insight into new potentially preventable or treatable risk factors for HIV transmission in MSM to reduce the high prevalence of HIV/AIDS in this population.